1. Introduction
This invention relates to positive working photoresists that are especially useful for high temperature applications, and more particularly, to photoresist compositions comprising a positive working light sensitive compound in an alkali soluble binder comprising a homopolymer or copolymer of a vinyl phenol, to a method for obtaining high temperature resistance and to articles formed therefrom.
2. Description of the Prior Art
Photoresist compositions are well known in the art and are described in numerous publications including DeForest, Photoresist Materials and Processes, McGraw-Hill Book Company, New York, 1975. Photoresists comprise coatings produced from solution or applied as a dry film which, when exposed to light of the proper wave length, are chemically altered in their solubility to certain solvents (developers). Two types are known. The negative acting resist is initially a mixture which is soluble in its developer but, following exposure to activating radiation, becomes insoluble in developer thereby defining a latent image. Positive-acting resists work in the opposite fashion, light exposure making the resist soluble in developer.
Positive working photoresists are more expensive than negative working photoresists but are capable of providing superior image resolution. For example, the positive working photoresists described above can be developed to yield relief images with a line width as low as 1 micron or less. In addition, considering the cross-section of a photoresist image, the channels formed in the resist by development have square corners and side walls with only minimal taper.
The positive-working photoresists comprise a light sensitive compound in a film-forming polymeric binder. The light sensitive compounds or sensitizers, as they are often called, most frequently used are esters and amides formed from o-quinone diazide sulfonic and carboxylic acids. These esters and amides are well known in the art and described by DeForest, supra, pages 47 through 55, incorporated herein by reference. These light sensitive compounds, and the methods used to make the same, are all well documented in prior patents including German Patent No. 865,140 granted Feb. 2, 1953 and U.S. Pat. Nos. 2,767,092; 3,046,110; 3,046,112; 3,046,119; 3,046,121; 3,046,122 and 3,106,465, all incorporated herein by reference. Additional sulfonamide sensitizers that have been used in the formulation of positive-working photo-resists are shown in U.S. Pat. No. 3,637,384, also incorporated hereby by reference. These materials are formed by the reaction of a suitable diazide of an aromatic sulfonyl chloride with an appropriate resin amine. Methods for the manufacture of these sensitizers and examples of the same are shown in U.S. Pat. No. 2,797,213 incorporated herein by reference. Other positive working diazo compounds have been used for specific purposes. For example, a diazo compound used as a positive working photoresist for deep U.V. lithography in Meldrum's diazo and its analogs as described by Clecak et al, Technical Disclosure Bulletin, Vol. 24, No. 4, September 1981, IBM Corp., pp 1907 and 1908. An o-quinone diazide compound suitable for laser imaging is shown in U.S. Pat. No. 4,207,107. The aforesaid references are also incorporated herein by reference.
The resin binders most frequently used with the o-quinone diazides in commercial practice are the alkali soluble phenol formaldehyde resins known as the Novolak resins. Photoresists using such polymers are illustrated in U.K. Pat. No. 1,110,017, incorporated herein by reference. These materials are the product of a reaction of a phenol and formaldehyde under conditions whereby a thermoplastic polymer is formed with a melting point of about 125.degree. C. Novolaks with melting points well in excess of 125.degree. C. are known but are not generally used in photoresist formulations because they are often brittle or have other properties limiting their use.
Another class of binders used with the o-quinone diazides are the homopolymers and copolymers of vinyl phenol. Photoresists of this nature are disclosed in U.S. Pat. No. 3,869,292 incorporated herein by reference. It is believed that photoresists using binders of polymers formed from vinyl phenols have not been used in commerce.
In the prior art, the above described positive resists using Novolak resins as a binder are most often used as masks to protect substrates from chemical etchants in photoengraving processes. For example, in a conventional process for the manufacture of a printed circuit board, a copper clad substrate is coated with a layer of a positive working photoresist, exposed to actinic radiation to form a latent circuit image in the photoresist coating, developed with a liquid developer to form a relief image and etched with a chemical etchant whereby unwanted copper is removed and copper protected by the photoresist mask is left behind in a circuit pattern. For the manufacture of printed circuit boards, the photoresist must possess chemical resistance, must adhere to the circuit board substrate, and for high density circuits, must be capable of fine line resolution.
Similar photoresists are also used in the fabrication of semiconductors. As in the manufacture of printed circuits, the photoresist is coated onto the surface of a semiconductor wafer and then imaged and developed. Following development, the wafer is typically etched with an etchant whereby the portions of the wafer bared by development of the photoresist are dissolved while the portions of the wafer coated with photoresist are protected, thereby defining a circuit pattern. For use in the manufacture of a semiconductor, the photoresist must possess resistance to the chemical etchant, must adhere to the surface of the semiconductor wafer and must be capable of very fine line image resolution.
A more recent development in the fabrication of semiconductors substitutes dry plasma etching for wet chemical etching to define a circuit. Plasma etching is the etching of a material by reaction with chemically active gaseous radicals formed by glow discharge. It provides advantages over wet chemical etching in that it offers process simplification and improved dimensional resolution and tolerance. Procedures for plasma etching are known and described in the literature, for example, Paulsen, Plasma Etching in Integrated Circuit Manufacture, J. Vac. Sci. Technical, Vol. 14, No. 1, Jan./Feb. 1977, pp 266 to 274.
Whether a semiconductor is manufactured using wet chemical etchants or plasma etching, photoresists are required to define an image pattern and to protect the surface of the wafer where etching is not desired. However, the demands on the resist are significantly greater when using plasma etching. For both wet etching and plasma etching, the resist must adhere to the substrate and must be capable of fine line image resolution. For plasma etching, in addition to these properties, the resist must often be capable of withstanding high temperatures without image deformation and without eroding as plasma etching generates high temperatures at the wafer surface.
The prior art positive working resists described above provide good resistance to chemical etchants and fine line image resolution. However, they soften and begin to flow at temperatures in excess of about 120.degree. C. In addition, these resists tend to erode when struck by the gas stream generated during plasma etching. This results in a rounding of the desired square corners of the resist image, a flow and filling in of the channels formed by development of the resist and a thinning of the resist layer, all of which result in image distortion and poorer image resolution.